The present invention relates to a basic subscriber access system for an ISDN (Integrated Services Digital Network) and, more particularly, to a simple adapter for multiplexing ISDN terminal equipment.
Wiring arrangements for an ISDN basic subscriber access system of the kind described are taught in CCITT RED BOOK VOLUME III-FASCICLE III.5 "INTEGRATED SERVICES DIGITAL NETWORK (ISDN)", Recommendation (Rec.) I.430 ("BASIC USER-NETWORK INTERFACE-LAYER 1 SPECIFICATION"), ANNEX A ("Wiring configurations and round trip delay considerations used as a basis for electrical characteristics"). Specifically, as shown in Rec. I.430, FIG. A-1/I.430., there is available a point-to-point configuration in which a terminal equipment (TE) constituting a single transmitting section and a network termination (NT) constituting a single receiving section are located at opposite ends of a single cable. Also available is a point-to-multipoint configuration which uses a short passive bus for allowing a maximum number of eight TEs to be connected to any desired points of a single cable, as shown in Rec. I.430, FIG. A-2/I.430. Another point-to-multipoint configuration is shown in Rec. I.430, FIG. A-3/I.430 which uses an extended passive bus, i.e., a passive bus is extended with points of connection of TEs to a cable being collectively located at the end which is remote from an NT.
A problem with the prior art ISDN basic subscriber access system is that, whatever its configuration may be, the number of TEs which can be identified is limited despite that theoretically a maximum number of 127 TEs are identifiable, because electrical characteristics have to be guaranteed. In this connection, in CCITT Rec. I.430, the point-to-multipoint configuration using a passive bus accommodates only eight TEs at maximum (see ANNEX A, "A.2.1.2"). Another problem is brought about when a call is received without any terminal being designated in the ISDN basic subscriber access system, i.e. reception on Global Address basis. Specifically, the Global Addressing allows all of the terminals which may respond to actually respond and, therefore, makes it impossible for a subscriber to limit the reception range beforehand in association with the connectors of S/T interfces. This undesirably causes a call to be received at all connector terminals (corresponding to CCITT Rec. I.430, point S) uniformly.